Welcome to iMore! Are you seeing the incoming call notification or are callers just being sent to voicemail? If you can see the incoming call try this:

1. Make sure your phone isn't in silent mode using the switch on the outside, left edge of the device.
2. Make sure you don't have Do Not Disturb enabled by looking for a moon icon in the upper, right corner of the screen, near the battery indicator. If DND is on, open Control Center and tap the DND button to turn it off.
3. While viewing your homescreen, tap the volume up button on the outside, left edge of your device to make sure you don't have the volume turned all the way down.
4. Finally, reset your device by holding the power button and home button down simultaneously until you see the Apple boot logo.

If you're still having issues after doing all of that, let us know and we'll go from there.